High voltage MOS transistors are semiconductor devices that can operate with high terminal voltages. High-voltage integrated circuits (ICs) that include high voltage MOS transistors are widely used in applications for the automobile industry, display drivers, portable telecommunication devices, medical equipment, and other areas. As an example, high voltage (e.g., greater than 20 volts) MOS transistors are integrated into a gate driver IC to deliver display signals to a liquid crystal display (LCD) panel. However, with the continuous process shrinking in advanced technology, breakdown voltages of these high voltage MOS transistors are also reduced.
Electric breakdown in MOS transistors is a well recognized problem. The breakdown phenomena generally escalate as the space between the source and drain regions continues to narrow in MOS transistors made by advanced processing technology, stemming from increased electric fields in the channel region. Known breakdown mechanisms in an MOS transistor include junction breakdown, punchthrough breakdown, and snapback breakdown. As an example, snapback breakdown occurs near the drain region in an NMOS transistor during saturated operation (e.g., the transistor is turned on). When a voltage is applied on the drain, a lateral electric field is presented in the channel region of the transistor and a peak electric field occurs near the drain region. The high electric field accelerates the electrons in the channel region and causes the electrons to gain enough kinetic energy to become “hot” near the edge of the N+ drain region. The “hot” electrons cause impact ionization of materials near the drain edge and create electron-hole pairs. Electrons will inject into the gate oxide, and some of the injected electrons may become trapped in the gate oxide layer. This so-called hot carrier effect may cause various problems in a MOS transistor. The trapped electrons may cause the threshold voltage (Vt) of an NMOS transistor to undergo an undesirable shift to the positive side. The hot carrier effect may also lead to long term device degradation and reduced reliability. Under extreme circumstances, snapback breakdown may even cause permanent physical damage in the gate oxide.